Can I add a sink drain where a toilet is already plumbed?


I added a picture that the previous home owner gave me before the slab was poured. They put in plumbing for a toilet (just in case they wanted one later) in this casita/workshop. Can I run my drain for a bathroom sink into the stack (drawn in red)? I know it would also need to be vented so the second question would be can I connect the vent to the plumbing vent as well (drawn in blue)? Everything I can find seems to point to yes, but I want to be sure before I put this in...

can i add a sink drain where a toilet is already plumbed
  12 answers
  • I would check with a licensed plumber to make sure everything is up to code.

  • I too would check with a licensed plumber to make sure everything is done correctly, properly, and to code so there are no issues.

  • Redcatcec Redcatcec on May 03, 2022

    Bring in several licensed plumbers and ask them, free estimates. Being up to code is very important

  • Mogie Mogie on May 03, 2022

    Another vote for first checking with a licensed plumber first. They might be able to spot and point out any potential problems. The most obvious peace of mind knowing this is done right.

  • Janice Janice on May 03, 2022

    Kudos to the previous owner who planned ahead. As other's have advised, I'd get a couple of licensed plumbers to take a look at it to be certain what you're planning to do is safe and up to local codes. You'll be ahead in the long run.

  • William William on May 03, 2022

    The toilet and sink drains need to be separate into the main drain. Does the red stack vent up through the roof? The sink needs to go into the main drain pipe not tied into the vent stack. If there was a blockage in either the sink or toilet it can cause a backup or draining problem. The blue vent for the sink is correct. You can consult a plumber or building department in your town hall.

    • Greg Greg on May 03, 2022

      Yes, I would run the red stack all the way through the roof since there are no other vents currently in that structure.

  • Kathy Gunter Law Kathy Gunter Law on May 03, 2022

    You should be able to use that drain. As for the vent, we used some of the "box" type where you don't have to run it out of the house and it has worked beautifully. It's a bit pricey for the part but it saved having to cut through the wall or ceiling.

  • It is best to check with a licensed plumber. That is to make sure everything is done properly.

  • Dee Dee on May 03, 2022

    You really need to check with a licensed plumber to make sure it is up to code. Your previous owners may not have done that. Your potty pipes and sink must be separated.

  • Betsy Betsy on May 03, 2022

    Hi Greg: If you can do it yourself, then I'd do it. But, and this is a BIG but, I think I'd have a professional do it. If something goes wrong and you're not licensed to do such work, your insurance won't cover it. Be sure that the contractor is bonded, licensed and insured. AND, a permit will have to be pulled. DON'T YOU PULL IT! If they tell you to pull the permit and you'll save some money, show them the door! Whomever pulls the permit says that they are doing the work, and if it goes pear shaped and you pulled the permit, you are saying you did the work and nothing is being covered by the insurance or maybe even small claims court if it gets to that. Also, your taxes may go up a bit. I don't want to rain on your parade, but I do want you to know these things. And, get several bids, check their references and put in your contract, and have a written, dated and signed by everyone contract, when the work is to commence and end. And, put in this sentence "Time is of the essence.", even if it isn't. That ties them down to a starting and ending date and not 2 or 3 years in the future or whenever they feel like getting around to finishing it. If they put up notices, which they should do, of what work they are doing, plumbing, concrete, electrical, etc., take clear pictures of them. Sometimes when you take them down, they tear or get icky from the weather.

    Good luck

  • Flipturn Flipturn on May 04, 2022

    Building codes most often apply to new builds only, and they can vary according to location.

    As part of the structure has already been erected, the project might be categorized as a 'grandfather build' only, and may not be bound to ahering to new build codes.

    • Greg Greg on May 04, 2022

      It has already passed final inspection. I was going to sheetrock it but wanted to make sure I had the drain tied in just in case I wanted to add a sink down the road (and possibly a toilet).

  • Flipturn Flipturn on May 04, 2022

    'Good to hear.